OK guys, educate an old school mentality

[Hawkeye]

The new 5.0 is advertised at 11:1 compression correct?
1) How can it run pump gas safely? as, I was always taught 10.5:1 was the limit of true pump gas applications.
2) Now I've read articles of boosting this engine, which would in turn increase the CR even higher. How is this possible with pump gas? My concern would be detonation.

I am in no way a "boost" guy and have always had NA carb'd engines, so I am trying to get my head around this.

Thanks

[PaceFever79]

My guess would be variable cam timing.

[sweeney1030]

thats correct

[Martin0660]

My guess would be variable cam timing.

Plus advanced engine management ... When someone says pump gas and 10.5:1, you have to understand what it might be based on ... an A9L computer, a mechanical advance dizzy??? You can get away with crazy effective compression on pum gas if you can manage the timing correctly, and you can manage effective compression with cam timing. Most of those blanket statements are based on what is close to you (5.0 with A9L)

WAY more to all of this that what I said here (head design, chamber design, piton desigh, etc, etc), but in it's most basic form, it's advanced engine management.

[biminiLX]

All true plus they have oil squirters to keep pistons cooler and more detonation resistence, knock sensors also.
-J

[Hawkeye]

Ok, so going on what you are saying.....

Variable timing would allow the detonation to occur before the stoke is at full compression? (I have always thought of timing as when the spark is fired)

I am running a 408 with a MSD dizzy, and AL9. On my compression stroke, I am achieving 10.4 :1 compression. If we assume that the timing is set to detonate at the exact moment of full compression, then I am safe to run pump gas.

If my car had variable timing, then I could change when the detonation occurs thus changing the CR?

Is this right?


Or..... is it only cam timing, where the timing of the opening and closing of the Exhaust/Intake valves changes? This would change the AF entering the chamber at full compression.


Or is it a combo of the 2?

[Martin0660]

Wow, this could get deep fast.

Cam timing (and events) affect cylinder pressure. Cylinder pressure is a combination of things.

You are looking at detonation wrong. In the truest scence, detonation is an uncontrolled event.

[Hawkeye]

OK, I missed saying that cam timing also allows CR to change because the exhaust valve may allow compression to escape, thus lowering the CR.


I am just thinking through this, and you guys have been big help.

Thanks

[Martin0660]

Here is a snip from the Megamanual ...

total advance at WOT: should be from ~24° to ~40° depending on your bore size and combustion chamber characteristics. Older design engines (i.e. push rods, domed pistons, etc.), and those with large bores (big blocks, etc.) need more advance, about 36 to 38°. Newer designs (4 valve/cylinder, swirl port engines, etc.), and small bores, generally require less, about 28 to 32°. Engines that have a lot of miles on them require less as well, because of oil leakage into the chamber. Lower octane fuel also requires less advance (it burns more quickly), so if you are running 87 octane, use a few degrees less total advance than if you are running 94 octane.

That is some good basic information to help your set up your timing tables. But as you can see, smaller bores, 4 valve heads, swirl technology (etc, etc) all add factors that let you ower the maximum effective timing (MET). So now add a bunch of that stuff together and you can still make power and run much lower timing ... of course with much lower timing, you guessed it, you can tolerate higher compression :D

Further to J's point about piston cooling jets (guys are adding them to endurance stuff all the time now), another snip on basic causes of detonation ...



If you experience detonation in your engine, then either:

Your mixture is too lean. Increase the numbers in the VE table at the point the engine detonates (or increase the Req_Fuel if the detonation occurs at all points). Also make sure your fuel pump is operating well. It may be supplying enough fuel at idle, but not supplying enough when the demand rises. Use the oxygen sensor readings to determine if you are lean,
You have oil leakage into the combustion chamber (check the plugs for signs of oil). This could come past the rings or seals (possibly worn, or from something that was forgotten during assembly, or there are blocked drain back passages in the head, or the rings have been damaged by detonation or over-revving), a 'leaky' PVC system, or from a poor intake manifold gasket seal allowing oil into the runners (on push rod V-engines),
Your spark advance may come in too far and/or too fast. Edit the spark advance table to lower the rate at which advance is added. You may also need to limit the total advance. Most engines will not require more than ~36° (possibly more for a flathead design, or a large open chamber, large piston dome design). Newer cylinder head designs and 4 valve/cylinders heads generally don't require a lot of advance,
You have spark plugs that are too hot (get a range or two colder), or they are incorrectly torqued (if they are loose, they over-heat because of poor thermal contact with the head),
Your thermostat is too hot (you can try as low as a 160°F thermostat for street use),
You fan is not working properly. Use an electric fan that comes on at 175°F (Many stock electric fans don't come on until 210°F to 220°F). Verify that your fan comes on at the appropriate temperature (if electric) and that the front of the radiator is clear of debris, or
You may have a lot of air trapped in the cooling system (drill three or four 1/8" holes in the thermostat's flange (not the manifold or housing, on the actual thermostat itself) to help bleed the cooling system while you fill it).
So again, combine the timing technoogy you can use with a modern design (low timing) with temperature managment, piston cooling jets, thermal coatings, spark plug design and placement, even alloy design for the materials, and you threshold for MET goes down even further. All of a sudden, you need VERY little lead (timing advance) and you get closer to being able to fire the plug when you want it, rather than way ahead of time when the mixture can pre ignight (detonation) ahead of time.

A whole other discussion is pre ignition versus detonation, but I'm willing to use it here as a compromise.

[Martin0660]

Again a very simplistic view of cam timing effects on this topic ...

Effects of Cam Advance

Advancing a camshaft from its original position causes all of these valve events to happen earlier in the cycle. A camshaft advance of 4 degrees will cause each opening and closing event to occur four degrees sooner than before, changing the ability of the cylinder to build pressure. For example, if the intake closing event is designed to happen at 55 degrees after bottom dead center (ABDC) it will now close at 51 degrees ABDC, or 4 degrees earlier. The same is true of the exhaust events--they will happen 4 degrees earlier, even though they don't happen until after top dead center (ATDC) of piston travel.

Effects on Cylinder Pressure
Although all valve events occur earlier, the greatest impact of an advanced cam is to close the intake valve sooner in the compression stroke. This means that a greater volume of air and fuel gets trapped and compressed before being ignited and has the effect of creating more torque (http://www.ehow.com/about_6547919_happens-advance-camshaft-timing_.html#) and power. However, if too much pressure builds up, extreme heat can cause pinging (detonation). Advancing cam timing can require the use of higher octane fuels in order to avoid this.

[wick]

Your looking into this too far. Just like why does positraction work...it just does... Quoted from Joe Dirt..